--TD 4.3.5 Sink Connect SNKAS Test
baselib.report(0x00, "TD4.3.5 beginning.")             --report：参数1是上报的消息的MsgType字段，参数2是content内容










1. CVS does not present Vbus for the duration of this test.
2. CVS presents VCONN to the PUT CC pin terminated with Ra
3. CVS and PUT negotiate a PD Explicit Contract
4. CVS initiates negotiations for an Alternate Mode (Discover ID, SVIDs and Modes)
5. CVS verifies PUT supports an Alternate Mode [TD.4.3.5.V.1]
6. CVS initiates an Enter Mode command with a PUT supported Mode
7. CVS verifies:
a. PUT follows all electrical constraints for SBU and any reconfigured pins (i.e. 
USB Safe States). [TD.4.3.5.V.2]
b. PUT sends ACK for Enter Mode [TD.4.3.5.V.3]
8. CVS initiates an Exit Mode command for the active mode
9. CVS verifies:
a. PUT follows all electrical constraints for SBU and any reconfigured pins (i.e. 
USB Safe States). [TD.4.3.5.V.4]
b. PUT sends ACK for Exit Mode after successfully exiting the mode. 
[TD.4.3.5.V.5]
10. CVS removes VCONN
11. CVS verifies PUT transitions to Unattached.SNK within tVCONNOFF
a. All clocked signaling ceases [TD.4.3.5.V.6]